A. Field of the Invention
The invention relates to a network node for a wireless communication network, in particular for a wireless sensor network, which can communicate via a communication device with other network nodes in the sensor network.
B. Related Art
The network nodes employed in wireless communication networks and in particular in wireless sensor networks include corresponding communication devices for the wireless communication with other network nodes. Since network nodes in wireless communication networks often are operated self-sufficient by an energy storage device in the form of a battery or an accumulator, the transmit powers of the communication devices normally are low. The network nodes thus do not permit an RFID communication with a corresponding transponder, upon which normally higher transmit powers are employed in order to provide the transponder via contactlessly transmitted communication signals also with energy.
In the context of a near-field communication between an RFID communication device and a transponder, it is known from the print EP 1 820 139 B1 that the communication device is both operable in a communication mode and also in a detection mode. Within the communication mode the conventional RFID communication is effected via contactlessly transmitted communication signals, whereas in the detection mode detection signals are emitted with a lower transmit power than the communication signals in the communication mode. Here, in the communication device there is provided a receiving device for receiving detection signals reflected on a transponder, which receiving device evaluates the reflected detection signals and so detects the presence of the transponder in a defined area around the communication device.
From the print EP 1 988 487 A1 it is known to operate an IC card reading device in an energy-saving recognition mode, until a contactless IC card approaches the reading device, so as to then change it into a communication mode for communication with the IC card. Here, the presence of the IC card is detected via the monitoring of the energy consumption of the oscillator employed in the reading device, whereby an IC card in the environment of the reading device leads to a drop in the oscillator current.
From US 20080111661 A1 there is known an energy-saving method for establishing a communication between a reading device and a plurality of RFID tags located in the reading field thereof. The RFID tags, in principle, respond to an interrogation signal emitted by the reading device in certain time slots with tag identification information. In order to decrease the energy consumption for the interrogation procedure, the reading device increases the transmission energy, and thus the range, of the interrogation signal in several steps, starting with the lowest energy. The reading device determines the energy value to be chosen in particular according to an estimation of the number of RFID tags present.
In US 20070159331 A1 there is described a method for communication with objects which are equipped with an RFID unit. The RFID unit is operable in a first operating mode as a passive transponder and in a second operating mode as an active transmitting/receiver unit. For the second operating mode, the RFID unit has its own local energy supply, e.g. in the form of a battery. In principle, the RFID unit is operated in the first operating mode. Only when certain events occur, the operating mode is changed over to the second operating mode, in order to conserve the energy reserve.
From WO 2004/004196 A1 there is known a transmitting/receiving unit which in a first operating mode is operable as a Bluetooth device and in a second operating mode as an RFID unit.
From the publication “Efficient tag detection in RFID systems” by B. Garbunar et al, in “Journal of Parallel and Distributed Computing” No. 69, p. 180 to 196, 2009, there is known a proposal as to how there can be unequivocally and efficiently effected the recognition of transponders present in a field of an RFID reading device.
From WO 2008/065337 A1 there is known an RFID transponder, which has a control unit which switches the RFID transponder to an energy-saving mode when the RFID transponder is not located in the reading field of a transmitting/receiving unit.